In particular during hot briquetting with a roll press, some of the materials to be briquetted (for instance sponge iron) tend to stick to the roll surface or inside the molding pockets. It is true that the material to be briquetted first leaves the rolls as a continuous strip. The latter, however, sticks to the surface of a press roll and is deflected, thereby blocking the briquetting press in the end. Another drawback arising during briquetting is that fine-grained material is sintered on the roll surface due to a high pressing power and high temperatures, whereby the process sequence may also be disturbed. Furthermore, the roll surfaces wear in a disadvantageous manner. Sooner or later such problems will automatically lead to standstills in the installations, entailing corresponding costs. An apparatus for making briquettes from sponge iron is described in DE 35 07 166 A1.
It is already known in the case of briquetting roll presses that dry graphite powder is made to trickle onto the roll surface by means of a vibration feeder which is arranged above the press rolls and by means of a metering operation which is performed by a screw conveyor. It is true that there is already an improvement thanks to the use of graphite powder as the lubricant. However, there are ensuing disadvantages, in particular, because of the fact that the lubricant is distributed over the roll surface in a rather non-uniform manner and that the lubricant has relatively poor adhesion characteristics, so that considerable amounts are lost because of trickling. Since very hot materials are often processed by the briquetting roll press, there is a corresponding thermal current above the press rolls, whereby a considerable part of the graphite powder which is trickling down because of gravity does not at all reach the roll surface, but is directly discharged by exhaust air or suction to the outside. Hence, the use of such a dry lubricant for the above-mentioned purpose constitutes a considerable cost factor because the losses are relatively high. The graphite inside the exhaust air is deposited in a negative manner. For instance, the installations are coated with a black graphite film.
The generic U.S. Pat. No. 3,414,643 describes an apparatus for briquetting fine calcium oxide. The apparatus comprises rotating briquetting rolls whose surface is provided with molding pockets and is wetted by means of a nozzle device with an emulsion consisting essentially of water and a polyoxyethylated alkyl phenol. The briquettes are thereby meant to fall out of the molding pockets in an improved manner. However, this document does not show a method how the lubricant could be used more efficiently.